I hear a lot about nonlocality and it's relationship to QM

In summary: Bell Test ExperimentsIn summary, Non-locality is the concept that sending a cause faster than light is non-local and goes against relativity. However, there are complexities when it comes to interpreting quantum mechanics and its relationship with non-locality. Physicist John Bell devised a set of inequalities to test the predictive abilities of a local theory, and the majority consensus is that quantum mechanics is weakly non-local. There are active dissent movements discussing the results and what Bell meant by "local" and "non-local." Bell's paper can be found in PDF format at a provided link.
  • #1
actionintegral
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I hear a lot about nonlocality and it's relationship to quantum mechanics and relativity. But I don't know anything about it. Please give me a basic description.
 
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  • #2
The basic idea of non-locality is that sending a cause over a spacelike interval, or in other words faster than light, is non-local. If we accept relativity, this would be a no-no.

But beyond this, especially in connection with interpetations of quantum mechanics, there are subtleties. Decades ago the late physicist Johm Bell, at CERN, devised a set of inequalities on the predictive abilities that a local theory might have concerning a set of things with spacelike separation, and this made it possible to test experimentally whether quantum mechanics obeys those inequalities, and settle a question that goes back to EInstein's challnges to QM, whether QM is local or not. A long series of experiments have been done since, and the majority consensus is that it has been shown that QM is weakly nonlocal in that the predictive limits derived by Bell for local theories are violated, But the consensus denies that the strong sense of nonlocal I started off with applies to QM; correlation is not causation.

There are active dissent movements on these points and I'll let them speak for themselves. Possible points of dipute seem to be (1) Did the experiments really show what is claimed? and (2) Just what did Bell mean by "local" and "nonocal?"
 
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I would be grateful if someone could point me to a copy of John Bell's paper.
 
  • #4
actionintegral said:
I would be grateful if someone could point me to a copy of John Bell's paper.

You can find it at this link (my site), in PDF format (along with the key EPR & Aspect papers):

The Original References
 

1. What is nonlocality in the context of quantum mechanics?

Nonlocality in quantum mechanics refers to the phenomenon where two or more particles can be connected in such a way that the state of one particle can affect the state of the other, even if they are separated by a large distance. This is in contrast to the classical understanding of locality, where the state of a particle is only affected by its immediate surroundings.

2. How is nonlocality related to the principles of quantum mechanics?

Nonlocality is a fundamental principle of quantum mechanics, known as quantum entanglement. It is one of the key features that distinguishes quantum mechanics from classical physics, as it allows for the possibility of instantaneous connections between particles and the existence of superposition (where a particle can exist in multiple states simultaneously).

3. Does nonlocality violate Einstein's theory of relativity?

Nonlocality does not violate Einstein's theory of relativity, as it does not involve any transfer of information between particles at faster-than-light speeds. Instead, it is a consequence of the probabilistic nature of quantum mechanics and the collapse of the wave function when a measurement is made.

4. Can nonlocality be observed in experiments?

Yes, nonlocality has been observed and tested in numerous experiments, including the famous Bell test experiments. These experiments have consistently shown that the predictions of quantum mechanics, including nonlocality, are accurate and cannot be explained by classical theories.

5. What are the potential applications of nonlocality in technology?

Nonlocality has the potential to be used in various technologies, such as quantum computing and cryptography. It could also lead to advancements in communication and teleportation, although these applications are still in the early stages of research and development.

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